1. Field of the Invention
The present invention relates generally to a system for heating a non-metallic material and, more particularly, to such a system which is capable of rapidly heating a discrete layer of the material.
2. Description of the Prior Art
In recent years there has been an increasing trend in fabricating industrial components out of synthetic materials instead of metals and alloys. These materials can be tailored to meet special properties or combinations thereof be they physical, mechanical, electrical, or thermal. They can also be mass produced to suit automation techniques. For equivalent mechanical strength, their lower density results in light-weight structures. Moreover, highly complex parts, even with built-in metallic components when necessary, can be made with simple molding techniques at relatively lower processing temperatures than are usual in metal processing. Most of these synthetic materials are chemically inert. Multi-layered systems and composites can be made to satisfy very stringent specifications at a much lower cost. Since the performance requirements are quite diverse, most of these materials are advanced non-metallic, ceramic, and polymeric composite materials. Unfortunately, components made of these materials cannot be united by conventional methods that have been developed for metal components.
In particular, high performance composites used in aerospace applications have generally been thermoset-resin based, although thermoplastics are now coming into use. Thermoplastic materials offer considerable advantages over the frequently used thermosetting composites because they are cheaper, have greater damage tolerance, and can be molded and remolded.
Thermoplastics tend to be fairly plastic and chemically inert, and are in essence, long-chain polymers with simple monomeric units. When heated, these materials soften and can be easily formed into complex shapes. Marginally defective thermoplastic laminates can be reprocessed by heating and be brought up to specifications thereby reducing waste. In contrast, thermosetting materials undergo chemical reactions during part processing, and cure or harden by forming complex crosslinking bonds; consequently, remolding is impossible. Further, volatile gases generated during the initial curing process may be trapped within the material resulting in mechanical and structural weakness. Because thermosetting plastics were used in conventional composites, welding of plastics did not receive much attention in the past; joining was based on adhesive bonding and mechanical fastening.
The joining, removal and repair of such a class of materials is a challenge to presently used welding methods. Often the size and shape of the structure, whether the materials to be joined are similar or dissimilar, thickness of material, non-metallic materials, temperatures the materials can withstand, the heating of the whole structure rather than the area to be joined, removed or repaired and the introduction of foreign, often undesirable, materials are problems with currently used methods such as arc welding, ultrasonic welding, induction welding, laser welding, and the like. In this disclosure, we propose a user safe, portable microwave system that can be used on a variety of materials in a variety of applications.
Considering one possible application, plastic, vinyl, and ceramic flooring overlayments have been used extensively in homes, hospitals, and commercial buildings. They offer a durable and easy-to-clean surface that is comfortable to walk on yet pleasing to the eye. These flooring sheets are attached to an underlayment such as wood or concrete by special adhesives. Using existing techniques, the adhesives require a long curing time for a proper bond. With our microwave system, however, this curing time can be reduced, yielding more efficient floor applications.
Further, the conventional removal of existing flooring can present even greater problems. To remove flooring it is necessary to heat the adhesive layer to its softening point. The flooring material can then be lifted from the underlayment. Conventionally, the heat has been applied from the top surface by using gas blow torches or super hot air blowers. This unfortunately causes the flooring material to scorch, producing toxic fumes.
An even greater problem is experienced in removing asbestos layered flooring. Unless the adhesive is completely softened, the asbestos breaks apart before the adhesive layer separates. This releases deadly asbestos dust in the air, endangering the workers as well as the tenants. In an effort to reduce this risk, flooring is often soaked with water to reduce the dust created. Unfortunately, not only is this a great mess in which to work, but often results in water damage to the underlayment material.
Our invention avoids all of these problems. The microwave energy from our system penetrates the surface layer and preferentially heats the adhesive layer directly. This leads to greater adhesive debonding without surface damage. Further, due to the more effective adhesive softening, asbestos coated tiles can be removed without separating the asbestos layer from its protective backing. For important background information relating to the use of microwave energy in conjunction with composite materials, we refer you to the technical article "Microwave Joining and Repair of Composite Materials" by Vijay K. Varadan and Vasundara V. Varadan, Polymer Engineering and Science, Mid-April 1991, Vol. 31, No. 7, pp. 470-486. The entire disclosure of this technical article is incorporated herein by reference.